Data modelling and database design are two very different activities.
 
For data modelling, the question you are asking is :
1) What does the world being modelled look like ?  
   In particular, you are looking for similarities between things. 
   Then you identify a 'super-type' of thing which may have sub-types. 
    For example, Corporate Customers and Personal Customers

   If, for example, supplier contacts are conceptually different things from customer contacts, 
  then the answer is that they should be modelled separately.  
  On the other hand, if they are merely sub-sets of the same thing, then treat them as the same thing.

2) For database design, you are answering a different question:-
   how can I efficiently design a database that will support the functions of proposed application or Web Site. 
   The key task here is to identify similarities between entities so that you can integrate them into the
   same table, usually with a 'Type' indicator.
   For example, a Customer table, which combines all attributes of both Corporate and Personal Customers.
   As a result, it is possible to spend a great deal of time breaking things out when creating a Data Model, and 
   then collapsing them back together when designing the corresponding database.

   
Dan Horne writes about Case ...

"I have found Case Studio to be an outstanding tool in its market niche. 
It supports a wide variety of databases, although I've only use it with Oracle, 
where I have found it to be excellent.

It's a bargain at $299, and with additional licenses costing only $89, it is well 
within the budget of most organizations that do not require an enterprise solution. 

I've been searching for a data modelling tool for some time, but most are not within 
my price range, and are generally aimed at larger companies. 

The competitive products I have found are marginally cheaper, but do not support the 
fundamental features I need such as the reverse engineering of existing databases.

The only weakness is the documentation, which is included as a help file.

Some topics such as exporting the diagrams to a graphic file are not covered, and there is 
no detail on writing templates to enhance and extend the system. 

Likewise, Data Flow Diagrams are barely touched on.

Nevertheless, I am very pleased with my purchase. 

Case Studio deserves to be much better known than it is.

Regards

Dan Horne"